1. Field of the Invention
The present invention relates to a driving method for a vacuum fluorescent display, and to a vacuum fluorescent display in which the driving method is used.
2. Description of the Related Art
Besides ZnO:Zn (green), which has excellent luminescence characteristics, numerous types of phosphors in which In2O3 or another electrically conductive substance is added to SrTiO3:Pr (red), CaTiO3:Pr (red), Gd2O2S:Eu (red), Y2O2S:Eu (red), La2O2S:Eu (red), SnO2:Eu (orange), ZnS:Mn (orange), ZnGa2O4 (blue), ZnGa2O4:Mn (green), or the like have been researched and developed as phosphors for low-energy electron excitation in vacuum fluorescent displays and the like.
However, except for green-luminescent ZnO:Zn, phosphors that have been developed for low-energy electron excitation generally have a short service life.
The dynamic drive method is known as a method for driving a vacuum fluorescent display. In the dynamic drive method, when the duty cycle (hereinafter abbreviated as “Du”) is kept constant, the luminance is sometimes lower and sometimes substantially the same due to the variation of the pulse width tp. The Du in this instance is indicated as the ratio (tp/T) of the pulse width tp and the pulse repetition period T. The luminance is substantially the same in a phosphor having a high response speed, and the luminance decreases in a phosphor having a low response speed. The response speed is indicated as the time at which the phosphor reaches saturation luminance after a voltage is applied to the anode. A phosphor having a low response speed does not reach saturation luminance during voltage application, and therefore has reduced luminance. Dynamic driving using a phosphor with a low response speed is therefore considered to be disadvantageous for obtaining the necessary luminance (Japanese Laid-open Patent Publication No. 2000-250454).
Therefore, when a phosphor having a low response speed is used, the pulse repetition period T is set to 8 to 20 msec to avoid having the pulse repetition period T be shorter than necessary (i.e., to avoid shortening the pulse width). For example, when Du=1/10 to 1/50 and T=10 msec, the pulse width tp for driving is relatively long, being 200 to 1000 μsec.
The pulse repetition period T is preferably set to 10 msec or less to prevent flickering of the display screen, particularly when the vacuum fluorescent display is subjected to vibration or the like (T. Kishino ed., Vacuum Fluorescent Displays, p. 155, Sangyo Tosho).
However, when the pulse width is increased in dynamic driving as described above, display screen flicker and uneven luminance occur, which can lead to reduced display quality.
A method for enhancing the luminance life of the phosphor in dynamic driving is disclosed in Japanese Laid-open Patent Publication No. 2003-195818. An object of this method is to prevent light and dark areas of uneven luminance parallel to the cathode from occurring over time in a vacuum fluorescent display, in particular, a vacuum fluorescent display having a rib grid electrode. In this method, at least one of the pulse width and voltage of the drive pulse applied to at least one of the anode and the grid is adjusted in conjunction with the distance from the cathode to the anode, and the amount that the pulse width and voltage is adjusted in conjunction with the distance from the cathode increases with increased cumulative active time.
A vacuum fluorescent display driving device is also known that comprises driving means for dynamically driving a vacuum fluorescent display by a drive voltage, the drive voltage necessary for driving the vacuum fluorescent display being fed to the driving means; temperature detection means for detecting the temperature of the operating environment of the driving means; and voltage variation means capable of varying the anode voltage fed to an anode of the vacuum fluorescent display and bringing the voltage to the necessary voltage value from the drive voltage according to the result of temperature detection by the temperature detection means (Japanese Laid-open Patent Publication No. 11-95712).
However, various types of phosphors have been developed for low-energy electron excitation, and vacuum fluorescent displays that use these phosphors are in practical use. Most of the phosphors used in these vacuum fluorescent displays have low luminance and short service life even when the method of improvement described above is used, except in the case of the green phosphor ZnO:Zn. There is therefore a need for further increased luminance and service life in a vacuum fluorescent display.